The present invention is directed to coating powders which produce translucent coatings particularly suitable for coating light-emitting devices, such as incandescent light bulbs.
High-temperature, light-emitting devices, such as incandescent light bulbs are conventionally coated for translucency with liquid coatings comprising silicone resins in organic solvents. There is a general need to replace such hazardous and polluting coatings with non-hazardous and non-polluting powder coatings.
Powder coating compositions which yield transparent or translucent coatings are well known in the art. Typical organic binder materials for such coatings are epoxy, polyester, and acrylic resins. Unfortunately, coatings based on these organic binder systems darken and decompose upon prolonged exposure at typical incandescent operating temperatures of 300 to 700xc2x0 F. (149 to 371xc2x0 C.) and thus are not useful for light-emitting devices. Coatings containing blends of silicones and significant levels of organic binders, such as those disclosed in U.S. Pat. Nos. 5,684,066 to Eklund and 4,877,837 to Reisling also discolor and similarly are not useful for light-emitting devices.
Coating powders based totally on silicone resins or substantially entirely on silicone resins are known. For example, Daly et al. in U.S. Pat. No. 5,422,396 disclose in a xe2x80x9ccomparative examplexe2x80x9d a 100% silicone resin based on Dow Corning 6-2230 silicone resin. The xe2x80x9ccomparative examplexe2x80x9d formulation in Daly et al. contained 80 phr (parts per hundred resin by weight) mica. Although silicone-based powder coatings having as low as 40 phr mica or other reinforcing filler have been described, higher levels such as 60 phr and upward are generally used in high-temperature coatings. Below about 60 phr filler levels, coatings tend to be insufficiently reinforced for high-temperature use as such coatings tend to crack and peel at high temperatures.
Silicone coatings containing 60 to 80 phr and upward filler, such as mica and/or wollastonite (calcium metasilicate) are resistant to cracking and peeling at high temperatures. However, such fillers contribute to opacity and yellowing of the coatings, and filler levels of 60 phr give coatings which are too rough and too opaque to be generally useful for light-emitting devices.
It is a general object of the invention to provide coating powders for producing translucent coatings on high-temperature substrates, such as light-emitting devices including incandescent bulbs.
In accordance with the invention there are provided coating powders which provide high-temperature, translucent coatings useful for light-emitting devices such as incandescent bulbs. The coating powders comprise a binder, at least 90 wt %, preferably at least 95 wt % up to 100 wt % of which is a silicone resin. The coating powders are filled with between about 10 and about 50 phr, preferably between about 20 and about 40 phr, most preferably between about 25 and about 35 phr of fillers selected from the group consisting of needle-like calcium metasilicate, mica, rod-like glass particles, and mixtures thereof, the fillers having aspect ratios between about 8 and about 40, preferably between about 10 and about 25. The high aspect ratio of the fillers provide high-temperature reinforcement to the coatings at use levels sufficiently low to provide sufficient light transmittance for use with light-emitting devices.
The invention provides light-emitting devices, such as glass incandescent bulbs, having coatings derived from silicone-based coating powders, such coatings providing light transmittance at 1 mil thicknesses of at least 50%, preferably at least 80%.
The invention further provides coating powders having binders which are at least 90 wt % silicone, preferably at least 95 wt % silicone and which contain between about 0.05 and about 3 phr of zinc dialkylcarboxylate wherein the alkylcarboxylate ligands contain between 6 and 20 carbon atoms. Preferably, the alkylcarboxylate ions are branched at the carbon alpha to the carboxylate group. Such a catalyst is zinc neodecanoate.
Herein, unless otherwise noted, percentages are by weight. The resins in the coating powders are calculated at 100%, and other components are expressed as percentages relative to 100% resin.
Heretofore, fillers such as mica and calcium metasilicate used for high-temperature, silicone-based coatings had relatively low aspect ratios, e.g., 5:1 and 3:1 being typical. The aspect ratio of needle-like wollastonite and rod-like glass particles are the ratio of length to width or diameter. The aspect ratio of platelet-like mica particles is the ratio of the diameter to the thickness. In respect to aspect ratios, it is understood that these are average aspect ratios of the individual particles. Surprisingly, it is found that by using fillers with higher aspect ratios, enhanced high-temperature reinforcement if provided at lower filler use levels. High aspect ratio needle-like calcium metasilicate, rod-like glass particles, and mica, at use levels in the 10 to 50 phr, provide sufficient translucency to be used for coating light-emitting devices, particularly incandescent bulbs.
Coatings on incandescent bulbs formed from the coating powders of the present invention are typically about 1 mil (25 microns) thick, but may range from 0.5 to 2 mils thick. As a standard, therefore, at 1 mil thickness, the coatings formed from the coating powders of the present invention should provide at least 50% light transmittance, preferably at least 80% light transmittance, although for the thinner coatings it may be possible to,use more opaque coatings. The fillers allow sufficient light transmittance but do impart some translucency. For un-colored coatings, the coating powders of the present invention should provide, at least 80% light transmittance at 1 mil thickness. Colored coatings containing high-temperature stable pigments will transmit less light, depending upon the type and amount of pigment.
Silicone resins self cure by the condensation of siloxyl (SiOH) end groups of silicone resins by the reaction: 
Accordingly, the binder resin may be 100 wt % silicone without additional cross-linking agent. Silicone/glycidyl methacrylate coatings are described in Reisling (U.S. Pat. No. 4,887,837) and Eklund (U.S. Pat. No. 5,684,066) and silicone/acid-functional acrylic/triglycidyl isocyanurate compositions are described in Daly et al. (U.S. Pat. No. 5,422,396). The teachings of each of these patents is incorporated herein by reference. Organic resins in addition to the silicone resins are permissible in the coating powders of the present invention, providing such organic resins do not comprise more than 10 wt % of the binder system preferably no more than about 5 wt % of the binder system so as to minimize yellowing of the coating over time with high-temperature exposure. Such non-silicone resins include cross-linking agents such as those described above in U.S. Pat. Nos. 5,684,066 and 5,422,396 and resins added for purposes such as flow control, gas release, etc.
Preferred silicone resins have organic substitutents selected from the group consisting of phenyl, methyl, C2 through C6 alkyl and mixtures thereof. Preferred silicone resins have viscosities of between about 500 and about 10,000 cps at 150xc2x0 C., most preferably 2000 to 5000 cps. The preferred silicone resins have condenseable hydroxyl contents of between about 2 and about 4.5 wt %, preferably from about 2 to about 3 wt. %. Preferred silicone resins have glass transition temperatures (Tg) of about 55xc2x0 C. or above, preferably about 60xc2x0 C. or above. Preferred silicone resins contain about 0.2% or less of organic solvents, more preferably about 0.1% or less.
Coating powders in accordance with the present invention are formed in a conventional manner. The components of the coating powder are batched and shaken, e.g., for 5 minutes, to blend well. The materials are then extruded, e.g., at 100xc2x0 C. (230xc2x0 F.) in a Buss single screw extruder, allowed to cool, chipped, ground and screened to obtain a powder of appropriate size. Average particle size is typically 20-80 microns. Scalping at 100 mesh is typical to remove coarse particles. There is typically about 10% by weight of particles below 10 microns. The amount of material retained on a 325 mesh is typically between about 30 to 50 wt. %. The powder is then applied in a conventional manner, e.g., electrostatically, to a substrate. The substrate is heated at the time of application and/or subsequently so that the coating particles form a continuous film, and, especially in the case of isocyanate-cured silicone resin adducts, effect the cure.